The present invention relates to an apparatus and a method for forming a thin film which are suitable for forming a metal thin film or a semiconductor thin film which constitutes an IC, LSI or the like.
Various means for forming a thin film on a substrate have conventionally been proposed and various methods have been adopted therefor. An ion plating method, CVD and PVD, for example, are typical of those methods.
The ion plating method is a method of ionizing and depositing in vacuum the substance which is evaporated in an active gas or an inert gas by producing a high-frequency electromagnetic field between a source of evaporation and a substance being evaporated. The ion plating method includes DC ion plating in which a DC voltage is applied between the source of evaporation and the substance being evaporated. These ion plating methods are disclosed in, for example, Japanese Patent Publication No. 52-29971 (1977).
Commonly owned Japanese Patent Application Laid-Open (KOKAI) No. 59-89763 (1984) discloses an apparatus for depositing a thin film. This apparatus is provided with a counter electrode or a substrate holder for holding a substrate for deposition, and a grid disposed between the counter electrode and a source of evaporation which opposes the counter electrode or the substrate holder in such a manner as to be impressed with a positive potential relative to the counter electrode or the substrate holder. A filament for thermionic emission is further disposed between the grid and the source of evaporation. The grid is impressed with a positive potential relative to the filament to form a thin film. According to this structure, the substance evaporated from the source of evaporation is ionized by the thermions emitted from the filament. The ionized substance which passes through the grid is accelerated by the action of an electric field which is directed from the grid to the counter electrode or the substrate holder and impinges on the substrate, thereby forming a thin film having a good adhesion on the substrate.
There are many other methods and apparatuses for forming a thin film.
The conventional methods of forming a thin film, however, have some problems. A first problem is that the adhesion between the thin film formed and the substrate is weak, and a second problem is that it is difficult to form a thin film on a substrate having a low heat resistance.
Especially, in the case of the method disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 59-89763 (1984), a third problem is produced that when a high-melting substance is used, it is necessary to use an electron gun at the source of evaporation, which leads to a complicated structure of the apparatus and raised cost.
A fourth problem is that when an electron gun is used, the electrons spring out from the electron gun into the adjacent space, and even when the source of evaporation which consists of a substance being evaporated is used, if one terminal of the power source for heating the source of evaporation has a ground potential, the thermions spring out together with the evaporating substance, thereby influencing the plasma state so as to make the control thereof difficult.
Nonuniformity of a film thickness is a fifth problem. For example, in the case of forming a thin film by vacuum deposition, since the density of the particles flown from the source of evaporation to the substrate is not uniform, the distribution in the plane of the thin film formed is generally not uniform. Therefore, this method is unsuitable for the formation of a thin film which is required to have a uniformity in the film thickness, e.g., an optical thin film.
In order to ameliorate the nonuniformity of the film thickness, various methods have conventionally been proposed.
For example, there is a method of providing a plurality of sources of evaporation in order to improve the uniformity of the thickness distribution of a thin film formed on a substrate which is difficult in the case of using a single source of evaporation. Even the use of the plurality of sources of evaporation, however, cannot completely eliminate the nonuniformity of the thickness distribution. In addition, this method requires a comparatively high cost and, especially, in the case of using an electron beam EB as the source of evaporation, the cost of the apparatus is disadvantageously much raised.
There is also a method of disposing a plurality of substrates in such a manner as to have a curved surface at which the thickness of the thin film formed is equal, namely, an equal-thickness surface. Since such an equal-thickness surface is a curved surface, the method of disposing a plurality of substrates on the equal-thickness surface is effective when the size of the substrate is so small relative to the distance between the source of evaporation and the substrate that the equal-thickness surface is regarded as approximately a flat surface in the order of the size of the substrate. On the other hand, when the size of the substrate is large and the substrate is not in conformity with the equal-thickness surface (curved surface), this method is not effective.
In addition, there is a method of disposing a planetary jig for revolving the substrate opposed to the source of evaporation around one point in the vacuum container and rotating the substrate on its own axis while using another point in the vacuum container as the center thereof. This method requires a space in the vacuum container which allows the revolution of the substrate around one point and on its own axis, thereby disadvantageously increasing the size of the apparatus. Especially, in the case of using a substrate having a general curved surface or a large flat surface which does not allow for sufficient revolution of the substrate around one point and on its own axis in the vacuum container, it is difficult to make a thin film having a uniform thickness.
As a method of forming a zinc-oxide-base transparent conductive film such as a zinc oxide film and an aluminized zinc oxide film there are known ion plating, reactive sputtering, etc. The ion plating method is a method of ionizing and depositing in vacuum the substance which is evaporated in an active gas or an inert gas by producing a high-frequency electromagnetic field between a source of evaporation and a substance being evaporated, as disclosed in, for example, Japanese Patent Publication No. 52-29971 (1977). The reactive sputtering is known as reactive magnetron sputtering, as shown in, for example, "Nitto Giho" Vol. 24 No. 1, April (1986).
A sixth problem is that it is difficult to form a transparent conductive film by these methods while so controlling the high electric conductivity and the optical transmittance, which are the main film properties, as to have given values.